Windows 10 1903 keyboard not working & wont re-install

As part of reconditioning a HP Pavilion 15 laptop for resale, we cleared all user files, user apps, and updated windows 10 1903 on 29Sept. We did not keep windows.old regrettably.

We noticed after a few days that the numlock wasn’t working, so we did the following:

1. Check BIOS – no numlock option
2. Checked the registry for the appropriate settings – okay
3. Troubleshooting said no issues
4. Startup repair indicated no issues.
5. Uninstalled the keyboard via the device a manager, and rebooted. Keyboard not re-installed and now the whole keyboard isn’t working.
6. Tried external keyboard – same problem
7. Keyboard works fine in BIOS = Windows issue?
8. If we turn on the on-screen keyboard, the laptop keyboard/numlock both work fine, until the next reboot.
9. Issue the same in safe mode & minimal boot

? Windows Registry Issue?

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Addendum:

1. Tried SFC /scannow - no errors (interesting keyboard works in command prompt); DISM fix doesn't work either.

2. Also tried the Navigate to the registry keyHKEY_LOCAL_MACHINE\System\CurrentControlSet\Control\Class\{4D36E96B-E325-11CE-BFC1-08002BE10318} fix - but doesn't solve the issue for us.

3. Last device event: device started i8042prt

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Problem solved!

How and why I will never know!

Start/settings/ease of access/keyboard/filters/ on-off this is to control repetitive key strokes - it was on. Turning it off resolved the problem.
The Filter key is supposed to stop repeating a key if held down for more than a split second.
I note that the filter key on on another HP win 10 1903 laptop functions as it should, so maybe there is a registry corruption/setting related to filter keys/keyboards? Be good to know which register key is at fault?

As part of reconditioning a HP Pavilion 15 laptop for resale

If you are preparing a computer for resale, Download the Windows 10 ISO image, burn the Image to USB Flash drive using the Media Creation tool, boot off a Windows 10 Installer, and do a Clean Install of Windows 10. You would not have to clear user data and programs or upgrade to latest as you would install the latest Windows. All settings would be set to default.The Windows 10 product key is saved in the Bios, so you don't have to type it in to Activate Windows 10